Vitamins A and B12 particularly well represented. I believe it improves my mood too.
My next plan of attack with regard to making liver taste better is to try koshering it. The process is to take the meat, soak it in water for thirty minutes, pat dry and then cover in salt and let sit for one hour. Then, rinse off the salt, pat dry again, and cook. I figure this will get the blood out (I don't really need the iron) and make it taste less like liver. Other than that, I use A LOT of onions. I was using chicken livers because they are milder in flavor, but they come in 1.25lb containers and I find trying to deal with that much at once is something of a trial.
Anyway beef heart is actually pretty easy to incorporate into your diet- use it to make chili.
The kidneys at my local store just seem too big to buy; I'd rather not be stuck with quite that much if I can't eat it.
I don't have access to all the possibilities; they only put so much out in the store, and I can't quite handle the look of beef tongue.
Beef bone marrow, is often easy to get, however, because they sell soup bones with large cross sections of the marrow exposed.
I often just throw them into the crockpot, though sometimes I experiment because, ideally, you want to be able to scoop out the almost melted bone marrow with a spoon, but in the crockpot it usually just melts out into a broth.
I do wish there was a restaurant nearby that specialized in this stuff. It's nice to see what a professional does with it; that way I could have a better picture of how it's supposed to turn out.